This proposal represents the initiation of a long-term study of bacterial interactions in the well characterized cactus-microorganism-Drosophila model system. This system has been studied extensively, particularly the evolutionary ecology of cactophilic Drosophila, saprophages which breed in the necrotic stems of giant columnar cacti and feed on the resident microflora. Knowledge of the role of bacteria has lagged far behind the other components: cactus chemistry, yeasts, and Drosophila. Yet, bacteria play a primary role since they interact with all levels of organization. Effects of the bacterial community in cactus necroses may be manifested in nutritional quality, production of attractants and oviposition stimulants, and chemical modification of the cactus tissue. The three main objectives of this study on the bacteria associated with decaying stems of columnar cacti are to investigate the role of bacteria in rot initiation and the production of attractants, to determine how bacteria modify their chemical environment in cactus tissue, and to ascertain the nutritional value of bacteria to the Drosophila. This study will contribute to a basic understanding of the interactions of bacteria with the cacti, yeasts, and Drosophila. Knowledge of these interactions is essential to the study of ecological genetics and evolutionary biology of the yeasts and Drosophila and the evolution of the desert system as a whole. It is well known that insects are human competitors. They can carry disease and injure agricultural crops. Many attempts at biological control of insect pests have failed miserably because knowledge of the biology of the system was incomplete. The research proposed herein will extend the knowledge of insect population biology--how insects select host plants, their adaptative strategies, and genetic structures. In addition, the majority of identified bacterial species isolated from necroses are opportunistic human pathogens (e.g. Klebsiella pneumoniae). These bacteria, as well as other agriculturally important species (e.g. Erwinia spp.), are being vectored by resident insects.